Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into hydrocarbons. The term “hydrocarbon” as used in this specification encompasses not only molecules containing only hydrogen and carbon, but also molecules containing hydrogen, carbon, and other atoms, such as oxygen, sulfur, and nitrogen.
This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons. More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes and aliphatic alcohols. Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream.
Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors. Gas-agitated multiphase reactors, sometimes called “slurry reactors,” “slurry bubble reactors,” or “slurry bubble column reactors” operate by suspending catalytic particles in liquid and passing a feed stream of gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the slurry by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Gas-agitated multiphase reactors or slurry bubble column reactors (SBCRs) inherently have very high heat transfer rates; therefore, reduced reactor cost and the ability to remove and add catalyst online are principal advantages of such reactors in Fischer-Tropsch synthesis, which is exothermic. Sie and Krishna (Appl. Catalysis A: General 1999, 186, p. 55) give a history of the development of various Fischer-Tropsch reactors and the advantages of slurry bubble columns over fixed bed reactors.
Heretofore, it has been common to provide relatively large catalyst particles, e.g. particles having a Reynolds number greater than 0.1, particularly because larger particles facilitate filtration. However, because larger particles tend to settle faster than smaller particles, it is more difficult to provide a fluidized bed, with the result that the gas and liquid flow parameters are constrained. In slurry bubble column reactors where plug flow is desired, this results in a narrow window for the gas flow because it must be sufficient to generate enough liquid turbulence to distribute the catalyst particles but not so high that it results in a well-mixed flow regime.
U.S. Pat. No. 5,348,982, which is the same disclosure as EP 0450860 A2, describes a method for operating a slurry bubble column reactor to maintain plug flow over the column length. The application discloses converting catalyst particles to a desired particle size range, if necessary, of nominally 1–200 microns average diameter. The methods disclosed for this conversion include crushing or ultrasonic treatment. The disclosure teaches that the material may then be sieved, if necessary, to produce a powder that is predominantly within the desired particle size range. The disclosure teaches that catalyst particles below 5 microns should be avoided and that a more preferred diameter is greater than 30 microns.
U.S. Pat. No. 6,348,510 discloses an optimized process for producing heavy hydrocarbons according to the Fischer-Tropsch method. The method requires that solid catalyst particles have a Reynolds number greater than 0.1. According to the examples cited in the disclosure, a Reynolds number greater than 0.1 may require particle sizes greater than 38–60 microns depending on various properties of the suspending liquid and the catalyst particles themselves. This requirement is tied to improving the efficiency of the separation of the liquid phase from the solid catalyst particles. The disclosure defines Reynolds number in terms of average particle diameter, which is not defined.
Such large particles are disadvantageous when used with the present catalysts because the activity of the present catalysts is such that their placement on large-diameter particles results in a reduction in the overall use of the catalyst. More specifically, when the size of a catalyst particle having a given activity is increased, it will reach a size beyond which the catalytic sites will not be wholly utilized because the diameter of the particle will prevent reactants from reaching catalytic sites in the interior of the particle. Thus, a better range of catalyst particle sizes and a better system for characterizing optimum catalyst particle size are necessary. Moreover, a better method for choosing particle size ranges that reflects actual operating conditions is necessary.